Networks: LANs, WANs, TCP/IP
Lecture 2
What is a network?
A network is a collection of computers connected together with special hardware (and software to manage the network). What is the purpose of a network? To share resources
(Example of computers in the atrium that share printers and that share application software; example of business network sharing access to a database. printer is connected to a local machine where that machine is the server.)
client/server model: the client requests a service, the server "serves" or provides the service. One server – many clients.
Ex. When access a webpage on our browser – client. A web server provides information on the internet by housing publicly accessible web pages.
The first computer networks were geographically close to one another, often within a single building.
This is called Local Area Networks (LANs).
These networks were used by large companies for in-house data processing.
Universities used LANs for administrative, educational, and research purposes.
LAN technology use hardware to interconnect multiple computers. If one computer connected to the LAN is down the other computers can still communicate.
In many LAN systems, a cable connects each computer to a small electronic device called a hub that forms the “center” of the LAN (picture page 49, comer). Each computer attaches to the hub with a cable.
In time, university research LANs and commercial research LANs began to create communication links so that computers in different LANs could share information.
Then government networks and corporate networks joined. Eventually commercial networks were created for the sole purpose of giving consumers access to this rapidly expanding infrastructure, this network of networks and its associated computer-based communications.
Different types of networks: LANs, WANs. LANs typically connect computers within a small area, such as a room or part of a building - often use cable, can be wireless, so will only be feasible in a restricted area.
Various LAN technologies are completely incompatible. Multiple LANs cannot be plugged together. Each LAN technology has its own specifications.
WANs (Wide Area Networks) are used to connect wider distances so different hardware is used (e.g. fiber optic cable, leased long-distance phone lines)
Ex. Wans use modems, etc.
Motivation for the Internet
As computers became smaller and more affordable, companies and organizations acquired more computers -> began to see need for networking -> different divisions or departments independently made different decisions about networks -> companies and organizations began to feel the need for internetworking - allowing communication between networks so that one could communicate between computers connected to different LANs with in the company or from a computer on a LAN to the company's WAN.
LANs and WANs were incompatible in many cases due to differing protocols
(Examples: BrooklynCollege has many LANs - sci network for faculty in the science departments; networks of computers for student use in the Atrium and Library Cafe (separate networks); administrative network)
The government, especially the military, realized early on that there would be a need for interconnecting different types of networks. The Department of Defense funded research on studying how to achieve internetworking. It was that project that eventually evolved into the Internet.
The military had a particular perspective on internetworking. There was a fear of creating a network that would be dependent on a central "super computer" or central server. From a military perspective, that was dangerous, because a central computer might be wiped out during a military attack. So the research was oriented towards designing a robust internetwork that would be able to remain functional even if parts of the network would be - literally - destroyed in war. The Internet developed as a decentralized distributed network, with many servers.
The major product of the research on internetworking was the development of a standard for a communications protocol that could be used to interconnect different networks. A communications protocol is a standardized set of rules regulation communication between computers.
A standard protocol written for communication across WANs. The Internet can be viewed as a WAN. TCP/IP allows previously incompatible systems to communicate over a WAN.
The communications protocol used on the Internet is called TCP/IP. TCP/IP is software that is used to enable a computer to communicate with other computers on the Internet. Any computer that is connected to the Internet must have TCP/IP software running on it.
The manner in which messages are sent and received over the Internet is defined by TCP/IP (Transmission Control Protocol)/(Internet Protocol)
When you send information across the Internet from your computer to another computer, the data is broken into small packets. Routers send each packet across the Net individually. After all the packets arrive at the receiving computer, they are recombined into their original, unified form.
Internet packets are called IP datagrams.
As it suggests, the Internet handles datagrams in the same way that a telegraph office handles telegrams. Once the sending computer creates a datagram and starts it on a trip through the Internet, the sender is free to resume processing in the same way that a person is free to perform tasks after sending a telegram. A datagram travels across the Internet independent of the sender, just as a telegram travels to its destination independent of the person who sent the message.
IP – routes the data (special purpose machines called routers receive the packets access the routing information and pass the packets on towards their destination.
TCP – breaks the data into packets and recombines them on the computer that receives the information.
Packets contain fewer than 1,500 characters. Plus header which contains order in which packets should be assembled, etc. Each packet put into IP envelopes which contains addressing information (where to send the data). When all them are received TCP assembles them.
To make datagram routing and delivery possible, each computer attached to the Internet must be assigned a unique address.
IP addresses / DNS names
The cornerstone of TCP/IP is IP addressing. Every computer that is permanently connected to the Internet is given a unique number, called an IP address. (Similar idea to the way Social Security numbers are used to identify people.)
Format of IP address - 146.245.1.4 - 4 groups of numbers, separated by periods. Each number 0-255 (8 bits). (Explain that the total number of IP addresses is 256**4.)
BrooklynCollege has all the IP addresses beginning with 146.245.... The sci network is 146.245.1.x ; its network is 146.245.2.x There can be at most 256 computers on the sci network. Similar for the sci network.
People don't like to have to remember numbers. We like to use names. The DNS - Domain Name System is used to give names to computers on the Internet.
Sample DNS name: merupu.sci.brooklyn.cuny.edu
Different parts separated by periods. There is NO correspondence between these words and the numbers in the IP address. There may be more than 4 or less than 4 parts to a DNS name. IP addresses always have 4 parts.
A DNS name can be "read" by going from the most specific to the more general, similar to the way a mailing address works. We might send mail to
John Doe
123 Main Street
Smalltown, ST
USA
Think of the periods in a DNS name playing the role of a "new line" in an address. The DNS name merupu.sci.brooklyn.cuny.edu refers to a computer called "merupu" which is part of the "sci" network, which is part of the BrooklynCollege network, which is part of the CUNY system, which is an educational institution.
In the case of a DNS name, the regions are not geographic. The top-level domains are not states, but categories such as education (.edu), government (.gov), commercial (.com), organizations (.org)
com / Commercial organizations
org / Nonprofit organizations
mil / U.S. military
gov / U.S. government
net / Network providers and businesses
These are primarily used by American companies and organizations. Computers in other countries often identify by country-specific top-level domains, (introduced in us)
When we send mail in the US, we don't need to specify USA as part of the mailing address. However, if we send mail to another country, we do add on another line with the name of the foreign country. Similarly, there are country codes that are used as top-level domains, usually 2-characters, such as .ca for Canada, .jp for japan, .ru for Russia, .il for Israel.
Internet software automatically - behind the scenes - converts DNS names into IP addresses. So when we specify a location in a Web browser (or in an email address), the software first looks up the IP address and then initiates the connection. (Analogy to CEO asking a secretary to get someone one the phone - the secretary has to first look up the phone number and then make the phone connection.)
(Look at syntax of email address,)URL - uniform resource locator .
URL is broken into 3 parts:
1. - specifies the protocol to be used when retrieving this resource.
HyperText Transfer Protocol (Http) – The protocol that determines how messages exchanged between browsers and servers are formatted.
Web browsers fill it in, because http is the default. (Analogy to business card that might say Tel: 718 951-5657, Fax: 718 951-2222. If the "Tel:" or "Fax:" is omitted, we would assume telephone.)
(Figure Page 54, Reed)
2.Second part - DNS name of server -
3.Third part - name or path to the file on the server. May be case sensitive because on some computers, file names are case sensitive (i.e., there can be two different files, on called FILE1.TXT and one called file1.txt) This is a common source of confusion for people used to Windows, which is not case sensitive.
Email Address:
Email addresses – consists of a string of characters separated into two parts by the at sign character, @.
The prefix of the mailbox address identifies the user,
and the suffix gives the domain name of the computer on which the user’s mailbox resides.
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